The present invention is directed to a drive control for a drilling device, particularly drilling devices for operating diamond-tipped drills.
In such a drilling device, the drive motor is usually a series wound universal motor.
The characteristic line (speed over torque) of such a drive motor has a curve with the speed decreasing in a monotonic manner, that is, linearly to a great extent, as the torque increases. The curve of this characteristic line is shown in the drawing.
In a known drive motor of this type, stoppage is reached at approximately four to six times the nominal moment. Accordingly, the motor develops relatively high forces before stoppage and receives a correspondingly high current. When these conditions exist, it can lead to serious damage of the drilling device and the drilling tool.
In addition, the idling point of this drive motor runs at such a high speed, which decreases relatively rapidly during loading, that as a practical matter it cannot be used effectively. Moreover, the high idling speed results in considerable stress on individual components, such as the gear unit bearing and shaft seals, and determines the effective lifetime of the driling device. Furthermore, under certain circumstances, the high idling speed substantially influences the maximum noise level of the device during operation.
It is also known to provide a speed control connected with a current limitation for such a drilling device. The speed control prevents an unwanted increase in speed in the range of the partial load so that the device can be operated at the same speed. The current limitation limits maximum current absorption and the maximum torque whereby, when a determined current is exceeded, it reduces the voltage available at the motor so that there can be no further increase in current. As a result, the speed decreased very rapidly, possibly to a point of stoppage, when the moment requirement at the drill spindle is unchanged or increases. Since the current flowing under such conditions can be dangerous for the motor, because of the absence of cooling air at the stoppage point, a characteristic line which results in an additional reduction in current as the speed decreases is often selected for current limitation. Such a characteristic is also known as a "foldback characteristic line" in technial circles.
For the operator of a drilling device using diamond-tipped drilling tools, operated as a rule in the same manner as an upright drilling machine with hand actuated mechanical forward feed, the result of current limitation of the latter type, is that when the maximum contact pressure force is exceeded, which is equivalent to exceeding the maximum current, the drill spindle speed decreases very rapidly towards zero so that there is hardly time to react, that is to disengage or release the forward feed before the drill spindle stops. It is possible that the drilling tool or bit may become fixed so that the forward feed must be reversed considerably to place the drive in forward motion again. Such operational problems would be considered disadvantageous by a user of the drilling device.
The problem connected with the stoppage of the motor has special significance in that a virtually load independent speed is achieved over the entire work range with a speed-controlled drive. As a result, the operator loses the "feel" for the adjusted load state, because the entire speed dependent noise frequency spectrum takes on a "constant character" and the usual accustomed decrease in speed, which acts as a "load indicator" is no longer noticeable as a noise frequency decrease.